CMOS (Complementary Metal Oxide Semiconductor) technology is a relatively cheap solution, e.g. compared with III-V compound semiconductor technologies, such as GaAs, InP, and InGaP. For mobile wireless communication equipment, such as a mobile handset or other user equipment (UE), it could thus be desirable to employ CMOS for the radio frequency (RF) front-end circuitry, such as power amplifiers (PAs). However, there are some challenges that have to be sorted out. One issue is that because the breakdown voltage for CMOS devices is relatively low, it is difficult to have a CMOS PA that can deliver enough RF power required for the radio communication standards. LDMOS (laterally diffused metal oxide semiconductor) transistors, having higher break-down voltage than normal MOS transistors, and can be used in RF/microwave power amplifier design. Another issue is that, due to a high peak-to-average power ratio (PAPR) of OFDM-(Orthogonal Frequency-Division Multiplexing)-based modulation schemes, like 3GPP (3rd Generation Partnership Program) LTE (Long Term Evolution), an average output power level of a PA normally is backed off from the peak output power level by 6-9 dB. Therefore, the power efficiency of the PA is reduced substantially. To reach high power efficiency, Doherty-type PAs are used in base station PA design operating at a specific frequency band. However, for a mobile handset (and other relatively small equipment), this is not as feasible e.g. due to the limited available space, which can make it difficult to implement a quarter wave length transmission line, and the difficult to tune the delay/phase shift of the transmission line or passive components in a wide frequency range which a UE generally must cover.
A solution that can be used in such smaller equipment is the so called distributed active transformers (DAT), which combine the outputs of the multiple uniformed small PA units into one output and thus reduce the peak power level required for each PA unit. By reconfigurable arrangement, the DAT can be adapted to different power levels. An example of such a circuit is e.g. disclosed in Kim J et al. “A fully-integrated high-power linear CMOS power amplifier with a parallel-series combining transformer”, IEEE Journal of Solid-State Circuits, Vol. 47, No. 3, pp 599-613, 2012 (In the following referred to as “Kim et al”).
It is generally desirable to further improve the power efficiency of PAs, e.g. to reduce the dissipated heat and/or to improve battery time for battery-powered equipment.